Retroreflective articles have the ability to return substantial quantities of incident light, which otherwise would be reflected elsewhere, back towards the light source. This ability has led to widespread use of retroreflective articles in a variety of applications relating to traffic and personal safety. For example, in the traffic safety area, retroreflective articles, such as pavement markings, have been used to help guide motorists.
Many flat pavement markings typically rely on an exposed-lens retroreflective optical system having light transmissible microspheres partially embedded in a binder layer containing reflective pigment particles such as titanium dioxide (TiO2) or lead chromate (PbCrO4). A portion of the microsphere that is not embedded in the binder is usually exposed to the atmosphere. In use, light from a vehicle's headlight enters the microsphere, is refracted towards the reflective pigment, and a portion is reflected by the pigment to return generally in the direction from which the light came. It is generally known in the art that the retroreflective performance of an exposed-lens flat pavement marking diminishes substantially when the microspheres becomes wet. This reduction arises because when the optical elements become wetted or covered with water, e.g. from rainwater, the ratio of the refractive indices at the elements' exposed-lens surface changes thereby affecting the elements' ability to gather incident light. Thus, some skilled in the art have pursued different methods to make articles exhibiting retroreflectivity under dry and wet conditions.
One method involves using a patterned pavement making, as described, for example, in U.S. Pat. No. 4,988,555 (Hedblom). In another method, U.S. Pat. No. 5,777,791 (Hedblom) describes an exposed-lens patterned pavement marking that exhibits retroreflectivity under dry and wet conditions by using a combination of microspheres having different refractive indices. A first type of microspheres, having a refractive index of about 1.9 to 2.0, contributes to retroreflectivity under dry conditions (referred to as “dry retroreflectivity”), while a second type of microspheres, having a refractive index of about 2.2 to 2.3, contributes to retroreflectivity under wet conditions (referred to as “wet retroreflectivity”). Others skilled in the art have also used combinations of microspheres with different refractive index to achieve dry and wet retroreflectivity. See U.S. Pat. No. 3,043,196 (Palmquist); U.S. Pat. No. 5,207,852 (Lightle et al.); U.S. Pat. No. 5,316,838 (Crandall et al.); and U.S. Pat. No. 5,417,515 (Hachey et al.).
Assignee's pending U.S. patent application Ser. No. 09/175,523 filed on Oct. 20, 1998, discloses pavement marking articles having enhanced retroreflectivity under wet conditions. The pavement marking has a monolayer of exposed lens optical elements that have an embedded surface, a spacing layer having two major surfaces, the first major surface in contact with the embedded surface of the optical elements, and a reflective layer on the second major surface of the spacing layer. A related case is U.S. patent application Ser. No. 09/175,857 filed on Oct. 20, 1998, which discloses a method of making a retroreflective element.
U.S. Pat. No. 2,354,018 (Heltzer et al.) discloses an exposed-lens light reflector sheet useful for making traffic signs or markers. In one embodiment, the reflector comprises transparent glass beads partially embedded in a pigmented bead-bonding coat, and a pigmented sizing film lying next to the most embedded portion of the glass beads and also in contact with the bead-bond. In another embodiment, the reflector sheet comprises, glass beads partially embedded in a bead-bond coat, a transparent spacing layer lying next to the most embedded portion of the glass beads and also in contact with the bead-bond, and a reflector (pigmented or aluminum foil) sizing coat lying next to the spacing layer. As shown in FIG. 3, the spacing layer is a substantially flat layer. The article may be very useful where incident light from a vehicle headlight strikes almost perpendicular to the surface of the sheeting. The article may not be as useful, however, where incident light enters the article at a high entrance angle because the spacing layer is not cupped around the embedded portion of the glass beads.
U.S. Pat. No. 5,812,317 (Billingsley et al.) discloses an exposed-lens retroreflective article exhibiting improved laundering durability useful, in one application, as reflective fabrics for clothing. The article has, in sequence, a layer of microspheres, a light-transmissible polymeric intermediate layer, a metal reflective layer, and a polymeric binder layer. By placing an intermediate layer between the microspheres and the reflective layer, enhanced laundering durability is provided without having substantial adverse affects on the retroreflective articles' optical performance under dry conditions, as measured according to ASTM E 810-93b. See column 3, lines 11 to 16 and column 8, line 18 to 19. The intermediate layer is preferably continuous, but there may be some very small regions—particularly at the most embedded portion of the microspheres—where the intermediate layer is discontinuous, i.e., its thickness is zero or approaches zero. See column 4, line 64.
Although the preceding technology is very useful in their applications, there still exists a need for a retroreflective article, in particular, an exposed-lens pavement marking, that exhibits dry and wet retroreflectivity.